I discovered another GIMP plugin, that created Voronoi diagrams. These might be more suited for creating mid sized streets in cities than mosaic filters and the such.
The plugin is here:
Attached is a sample created using the "random" grid and everything else default.
Even better than a random pattern would be one where you could set the nodes to represent population density. I found a java applet here
http://www.diku.dk/hjemmesider/studerende/duff/Fortune/ (bottom of page).
I have attached a sample of how you might seed this to represent a city in a radial pattern, but still random: